This invention relates to acidic electroplating baths and, more particularly, relates to the electrodeposition or electroplating of metals.
The electrodeposition of metals from aqueous solutions of the metal ions is well known. The present invention is concerned particularly with aqueous acid electroplating baths for depositing metals such as tin, lead, copper, zinc, cadmium or tin-lead mixtures. The composition of the metal electroplating bath will be determined at least in part by the particular metal to be electrodeposited. Various additives have been proposed in the art for improving the effectiveness of electroplating baths containing various metals.
In acidic electroplating baths, it is conventional to utilize a surfactant therein to control properties of the metal deposit and to assist in solubilizing other organic components. A wide variety of compositions have been suggested as surfactants in acid electroplating baths, and many of these are commercially available polyethylene oxide-based materials, and, therefore, typically contain a hydrophilic and hydrophobic portion within their chemical structure.
The present invention is directed particularly to improved aqueous acidic plating baths for depositing tin and/or lead coatings. Generally, these plating baths contain, in addition to the water-soluble tin and/or lead salts, at least one group selected from the group consisting of fluoborates, fluosilicates, sulfates, sulfamates, etc. In addition to these basic ingredients, the prior art has suggested improvements in such baths by including additives which will improve the brightness of the deposit obtained from such baths. In U.S. Pat. No. 3,875,029, the use of a naphthalene monocarboxyaldehyde either alone or in combination with certain substituted olefins described in the patent results in an improvement in the brightness of the deposit. Other ingredients which have been suggested as being useful additives in tin and/or lead plating baths include various combinations of carbonyl-containing compounds such as aldehydes, ketones and carboxylic acids, etc. For example, U.S. Pat. No. 3,749,649 describes the advantages of utilizing tin-lead plating baths containing at least one polyether surfactant and at least one aromatic aldehyde containing a chloro substituent. Another bath for producing bright deposits of tin-lead alloys is described in U.S. Pat. No. 3,785,939.
Typically, acid zinc plating baths have been based on a suitable inorganic zinc salt such as zinc sulfate or zinc chloride, and other additives to promote and improve ductility, brightness, throwing power and covering power. Surface active agents may be included to improve crystal structure, reduce pitting and increase the solubility of other additives.
Aromatic carbonyl-containing compounds generally are incorporated into acid zinc baths as a supplemental brightener additive and for improving fineness of the grain of the zinc deposit. Wetting agents or surfactants have been added to these baths to solubilize or improve the solubility of the carbonyl-containing compounds in the baths, but such wetting agents and surfactants generally result in a bath exhibiting a tendency to foam excessively, particularly on agitation with air.
A variety of plating baths have been developed and employed for electroplating cadmium onto metallic substrates. These baths typically utilize sulfates and cyanides as the primary electrolytes. The cyanide baths have proven effective and generally satisfactory despite certain objectionable features such as high toxicity, low current efficiency and hydrogen embrittlement of certain steels. The sulfate baths which have been suggested overcome many of the objectionable features of the cyanide baths. However, some of the sulfate-based baths contain such components as ammonium ions and chelating agents. Because of the ability of these agents to complex with heavy metal ions, there is a significant increase in the difficulty of eliminating heavy metals from spent baths. Sulfate-based baths which do not utilize ammonium ions or chelating agents have more recently been suggested in the prior art. For example, in U.S. Pat. No. 3,998,707, an aqueous acidic cadmium electrolytic bath composition is described which comprises cadmium ions, free acid, and a particular surfactant combination which comprises a cationic polyoxyalkylated amine and an anionic surfactant. Preferably the cadmium plating bath also contains at least one brightener. Examples of brighteners include aryl aldehydes such as anisic aldehyde, ring halogenated aryl aldehydes such as ortho-chlorobenzaldehyde, heterocyclic aldehydes such as thiophenaldehyde, aryl olefinic conjugated ketones such as benzylidine acetone, and heterocyclic carboxylic acids such as nicotinic acid. U.S. Pat. No. 4,045,305 describes cadmium plating baths which contain cadmium ions, free acid, a surfactant combination comprising a condensed naphthalene sulfonate compound and a non-ionic polyoxyalkylated surfactant. Preferably this bath also contains a brightener of the type described in U.S. Pat. No. 3,998,707.
Acid copper plating baths for producing a brilliant copper finish on articles have been known in the art, and a number of patents have described various brightening agents which can be added to acidic copper baths. Examples of such patents include U.S. Pat. Nos. 2,707,166; 2,707,167; 2,830,014; 3,276,979 and 3,288,690. In U.S. Pat. No. 3,725,220 it has been suggested that the utilization of organic sulfonates or carboxylates as brightening additives in acid aqueous copper plating baths results in improved stability of the bath and effective deposition of copper over a satisfactory current density range.
In a number of instances in the prior art acid copper plating baths, a sufficiently brilliant finish is obtained but little or no smoothing effect on the surface is obtained. The ability of a plating bath to produce deposits relatively thicker in small recesses and relatively thinner on small protrusions thereby decreasing the depth of surface irregularities is known as "leveling". For example, a copper plating bath with satisfactory leveling ability can be utilized to reduce or eliminate the effect of microscopic cracks or scratches on the surfaces of the articles being plated. Accordingly, a number of additives have been described in the prior art for increasing the leveling effect of acid copper plating baths. For example, U.S. Pat. No. 3,101,305 describes a leveling additive obtained from the condensation of thiourea with aliphatic aldehyde such as formaldehyde. Since the additives which have been described in the prior art are useful either as brightening agents or leveling agents, it generally has been necessary to utilize two additives in acid copper plating baths, one for brightness and another for leveling.